Bootstrap Protocol

The Bootstrap Protocol is a computer networking protocol used in Internet Protocol networks to automatically assign an IP address to network devices from a configuration server. The BOOTP was originally defined in RFC 951.
When a computer that is connected to a network is powered up and boots its operating system, the system software broadcasts BOOTP messages onto the network to request an IP address assignment. A BOOTP configuration server assigns an IP address based on the request from a pool of addresses configured by an administrator.
BOOTP is implemented using the User Datagram Protocol as transport protocol, port number 67 is used by the server to receive client requests and port number 68 is used by the client to receive server responses. BOOTP operates only on IPv4 networks.
Historically, BOOTP has also been used for Unix-like diskless workstations to obtain the network location of their boot image, in addition to the IP address assignment. Enterprises used it to roll out a pre-configured client installation to newly installed PCs.
Originally requiring the use of a boot floppy disk to establish the initial network connection, manufacturers of network cards later embedded the protocol in the BIOS of the interface cards as well as system boards with on-board network adapters, thus allowing direct network booting.
While some parts of BOOTP have been effectively superseded by the Dynamic Host Configuration Protocol, which adds the feature of leases, parts of BOOTP are used to provide service to the DHCP protocol. DHCP servers also provide the legacy BOOTP functionality.


The BOOTP was first defined in September 1985 in RFC 951 as a replacement for the Reverse Address Resolution Protocol RARP, published in RFC 903 in June 1984. The primary motivation for replacing RARP with BOOTP is that RARP was a link layer protocol. This made implementation difficult on many server platforms, and required that a server be present on each individual IP subnet. BOOTP introduced the innovation of relay agents, which forwarded BOOTP packets from the local network using standard IP routing, so that one central BOOTP server could serve hosts on many subnets.


Case 1 : Client and server on same network
When a BOOTP client is started, it has no IP address, so it broadcasts a message containing its MAC address onto the network. This message is called a “BOOTP request,” and it is picked up by the BOOTP server, which replies to the client with the following information that the client needs:
1. The client’s IP address, subnet mask, and default gateway address
2. The IP address and host name of the BOOTP server
3. The IP address of the server that has the boot image, which the client needs to load its operating system
When the client receives this information from the BOOTP server, it configures and initializes its TCP/IP protocol stack, and then connects to the server on which the boot image is shared. The client loads the boot image and uses this information to load and start its operating system.
The Dynamic Host Configuration Protocol was developed as an extension of BOOTP. BOOTP is defined in Request for Comments 951 and 1084.
Case 2 : Client and server on different networks
  1. Problem with the bootp request is that the request is broadcast. A broadcast IP data-gram cannot pass through any router. The router discards this packet.
  2. To solve this problem there is a need for an intermediary.
  3. One of the host or router can be configured at application layer to operate as relay agent.
  4. The relay agent knows the uni-cast address of bootp server and listens for broadcast message on port 67.
  5. When it receives this broadcast packet, it encapsulates the message in uni-cast data-gram and sends request to bootp server.
  6. The packet carrying a uni-cast destination address is routed by any router and reaches the bootp server.
  7. The relay agent after receiving the reply, sends it to bootp client.

    IETF standards documentation